In the prior art, two separate techniques have been utilized for the clinical treatment and physiological examination of lung function. The first technique measures pulmonary gas exchange and is referred to as the "multiple inert gas technique" (MIGT). Pulmonary gas exchange can be analyzed by the distribution of ventilation/perfusion ratios (V.sub.A /Q). In this type of analysis, the lung is conceptually divided into as many as 50 compartments, each having discrete V.sub.A /Q ratios. In practice, the distribution of flow and ventilation to the 50 compartments (the V.sub.A /Q ratio) is derived from the steady state elimination of six inert gases infused in solution at a constant rate. When the V.sub.A /Q ratio is expressed logarithmically, the distribution approaches a normal one such that the gas flow and ventilation can be described by their respective averages and standard deviations.
In 1977, Wagner and West published a computer program entitled "Pulmonary Gas Exchange" which calculated the pulmonary gas exchange from V.sub.A /Q distributions generated by the multiple inert gas technique (MIGT). The MIGT technique is not useful in acute clinical situations, because it produces a static picture of the pulmonary system. For example, the MIGT analysis does not permit prediction of changes, such as those caused by the administration of fluids and medications to the patient. Measuring pulmonary gas exchange to analyze respiratory problems, therefore, has a number of limitations resulting from its static nature.
A second method for analyzing the function of a lung is to evaluate pulmonary circulation, or the distribution of blood flow to the lung. This evaluation relies upon changes in blood flow with respect to changes in lung blood pressure. The pressure/flow relationship was first analyzed in detail by Fung, et al. in their study of the characteristics of cat lung tissue. Fung's measurements of all the properties and dimensions of cat pulmonary vessels showed that the nature of the pressure/flow relationship was attributable to the elastic nature of the vessel walls. The properties of the vessels were generalized and extended to provide a model of pulmonary circulation that permitted the generation of pressure/flow curves for any combination of pathologic or physiological changes (Marshall et al.).
However, the predominant issue in respiratory intensive care is the simultaneous achievement of adequate pulmonary gas exchange and steady-state hemodynamics. Abnormal values are expected and many of the therapeutic measures have been derived empirically and are applied by individual trial. The development of sophisticated monitoring tools has permitted success in the management of these complex disease states. However, the pathophysiological relationship between gas exchange and pulmonary blood flow remains obscure.
It is, for example, understood that because ventilation/perfusion ratios are determined by regional perfusion characteristics and that hypoxic pulmonary vasoconstriction (HPV) is the only known local vascular feedback control mechanism, HPV must actively regulate the V.sub.A /Q distribution. While this interaction has been recognized, no systematic quantification has been advanced for clinical or experimental applications.
Prior researchers have typically looked to the excessive number of influencing variables and retreated to interpret the changes in conceptual terms. These approaches have not included analysis of critical mechanisms, and have led to the acceptance of sometimes erroneous or misleading interpretations.
The present invention is directed to a novel analysis of the relationship between gas exchange and pulmonary flow as they impact on pulmonary intensive care. The present invention identifies the quantitative effects of the principal variables involved in pathophysiologic situations encountered clinically and particularly to evaluate the role of HPV in this relationship. The present invention combines the ventilation/perfusion ratio distribution with the pressure flow model to provide a more comprehensive method for analyzing lung function. The bridge between the above two inter-related aspects of lung function is HPV.
The present invention is specifically directed to the evaluation and treatment of patients undergoing acute pulmonary distress in an intensive care unit (ICU). Selected data from the patient's monitoring device are entered into a computer program in accordance with the present invention. The system analyzes all of the data and outputs both the best estimate of the underlying cause of the patient's distress as well as suggestions as to the next treatment step.